In a conventional technique, an energy management company that manages energy supplied to a plurality of buildings requests a reduction in power usage by each building in order to reduce the amount of energy use which is increased according to the season and/or time period. Each building presents the time period and amount of energy that can be reduced to the energy management company, and the energy management company reduces the amount of energy in accordance with the presented time period and amount of energy that can be reduced, as described in Japanese Laid-open Patent Publication No. 2005-107901.